<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .trapezoid {
      height:0;
      width:100px;
      border-width:0 40px 100px 40px;
      border-style:none solid solid;
      border-color:transparent transparent red;
    }
  </style>
</head>
<body>
  <div class="trapezoid"></div>
</body>
</html>